Multiple primate species, including 5 species of macaques (Macaca mulatta, M. nemestrina, M. fascicularis, M. assamensis, M. arctoides) and 3 species of langurs (Semnopithecus entellus, Trachypithecus phayrei, Trachypithecus pileatus) coexist with humans in different areas and ecological contexts (urban primates, temple primates, wildlife markets, pet primates, wild primates) in Bangladesh, one of the most densely populated countries in the world. This coexistence provides ample opportunity for human-primate interaction and poses a risk for cross-species transmission of infectious agents. Our proposed research will investigate the epizootiology, ecology and evolution of two simian retroviruses, simian foamy virus (SFV) and simian type D retrovirus (SRV) in macaques and langurs. We will also measure the prevalence of human infection with SFV and SRV among individuals exposed to macaques and langurs and describe how cross-species transmission may lead to viral recombination and evolution. All of these factors are important for the understanding of the potential emergence of new retroviral zoonotic disease(s).